1. Field of the Invention
The present invention relates to a plasma technology, and more particularly, to a plasma source for processing in the field of semiconductor fabrication technology.
2. Description of the Prior Art
The progress in the large scale integrated circuit (LSI) technology is bringing about a change comparable to that of the industrial revolution. A high packing density of the LSI has been realized through a reduction in the device dimensions, an improvement in device structures and enlargement of chip surface areas. In recent years, the device dimensions has been reduced to the light wavelength region, and the use of excimer laser or soft x-rays in lithography is being studied.
In the field of semiconductor fabrication processing, the plasma technology is widely used for dry etching, chemical vapor deposition, sputtering and so on. Especially, dry etching technique plays an important role in the formation of fine patterns, as well as lithography technique.
Dry etching is a process for removing unnecessary portions of the solid surface (e.g., a surface of a semiconductor substrate, layers deposited on a substrate, and so on) by utilizing chemical or physical reactions at the interface between plasma and the solid surface. The plasma-solid surface reactions are induced by the interaction between the solid surface and free radicals, ions, etc., generated in the plasma.
Reactive ion etching (RIE), which is the most widely used as a dry etching technique, removes the unnecessary portions of the surface of the sample by an etching reaction that occurs when the sample is exposed to the high-frequency discharge plasma of a suitable gas. The necessary portions are normally protected by a photoresist pattern used as a mask. In order to improve the fineness of the pattern, it is necessary to align the directionality of the ions. This cannot be achieved without reducing the ion scattering in the plasma. In making the directionality of the ions uniform, decreasing the pressure of plasma is effective in increasing the mean free path of the ions. However, it is difficult for a low pressure gas to be discharged by a high-frequency power (A.C. power). To solve this problem, magnetron reactive ion etching and electron cyclotron resonance (ECR) etching techniques, in which a magnetic field is applied on the plasma chamber, have been developed to generate a low pressure gas discharge.
FIG. 14 shows a typical prior art magnetron discharge etcher for reactive ion etching (RIE). In this etcher, a gas controller 82 passes into a metal chamber 81 and introduces the reactive gas, while the pressure is appropriately controlled by an exhaust system 83. An anode 84 is provided at the top of the chamber 81. A sample stage 85 (which serves as a cathode) is provided at the bottom. An RF power source 87 is connected to the sample stage 85 via an impedance-matching circuit 86 to facilitate a high-frequency discharge between the sample stage 85 and the anode 84. A rotating magnetic field is applied in the chamber 81 by a pair of opposing AC electromagnets 88 mounted on the sides and whose phases are shifted 90 degrees from each other, thus facilitating a discharge at a low pressure. The cycloid motion of the electron induced by the magnetic field improves ionization efficiency.
Although the purpose of using the above magnetron discharge and ECR discharge is to increase the plasma density, the discharges are still not sufficiently capable of generating uniformly a highly dense plasma over the entire surface of the sample. In the prior art magnetron reactive ion etching etcher, local plasma densities are regarded as uniform by means of a rotating magnetic field when averaging them over long time. In fact, however, the instantaneous plasma densities are not uniform. Therefore, when a wafer is exposed to the plasma for the fabrication of MOSLSI in the prior art etcher, local potential differences of the plasma may cause the gate oxide film of the MOSLSI to break down. In the ECR etcher, a magnetic field is distributed radially in the chamber, so that the local deviation in the plasma density occurs and it causes a non-uniform etching and local potential differences.
Due to the non-uniformity of the plasma generated in the prior art etcher, it is difficult to achieve a high-yield production of MOSLSI. Further, it is difficult to achieve a high repeatability in the etching of ultrafine-pattern LSI devices having thin gate oxide films and the etching of a large sample (e.g., a large-diameter wafer).
To generate a high-density plasma, and to lower the self-bias in the plasma for reducing damage to the sample caused by high energy ions, a high-frequency power (ranging from 100 to 200 MHz) may be superimposed with a 13.56 MHz power on the parallel plate electrode of the magnetron etcher. Even in this method, it is difficult to improve the uniformity of the plasma. Therefore, it is not sufficient to solve the problems caused by the non-uniform plasma.